Question

I am trying to intercept form submits from webpages I dont control.

My current implementation is …

// During onLoad, loop through all forms and for each form object
var prevonsubmit = formobj.onsubmit;
if (prevonsubmit) {
    formobj.onsubmit = function f() {
        if(prevonsubmit()) {
            interceptform();
            return true;
        }
        return false;
    };
} else {
    formobj.onsubmit = function ff() {
        interceptform();
        return true;
    };
}

The problem with this is, inside interceptform(), I am unable to identify which form actually made this submission. Is there a way I actually get the form object that is trying to submit? Keep in mind that some of the forms I see do not have a name or id specified and there is more than one form (in the same webpage) with same action.

Edit:
The purpose is capture the content in the input tags that belong to the form.
A made up example of what I see in a form:

<form action="https://duckduckgo.com/html/" method="GET">
    <input type="text" name="q"/>
</form>
<form action="https://duckduckgo.com/html/" method="GET">
    <input type="text" name="l"/>
</form>
<form action="https://duckduckgo.com/html/" method="GET">
    <input type="text" name="l"/>
    <input type="text" name="q"/>
</form>

Edit2:

Based on @ruakh answer, the solution I ended up using:

var prevonsubmit = formobj.onsubmit;
if (prevonsubmit) 
    formobj.onsubmit = createOnSubmitFunctionWithOld(prevonsubmit, formobj);
else
    formobj.onsubmit = createOnSubmitFunction(formobj);

// Definition of the functions:
function createOnSubmitFunctionWithOld(prevonsubmit,formObj) {
    return function () {
        if (prevonsubmit()) {
            interceptform(formObj);
            return true;
        }
        return false;
    };
}
function createOnSubmitFunction(formObj) {
    return function () {
        interceptform(formObj);
        return true;
    };
}

Answer 1

You can simply pass formobj as an argument to interceptform():

        interceptform(formobj);

But bear in mind that both with formobj and with prevonsubmit, you have to be careful to avoid capturing a variable you don’t want to. For example, in JavaScript, this:

var functions = [];
for(var i = 0; i < 10; ++i)
    { functions[i] = function() { return i; }; }

creates ten functions that all return 10, because they all capture the same i variable that’s been incremented up to 10 by the time the functions are ever called. In the above example, you could write something like:

function new_constant_function(return_value)
    { return function() { return return_value; }; }

// ...

var functions = [];
for(var i = 0; i < 10; ++i)
    { functions[i] = new_constant_function(i); }

to copy each value of i into a new local variable return_value whose value never changes; or, a bit more tersely, this:

var functions = [];
for(var i = 0; i < 10; ++i)
    { functions[i] = (function(_i){ return function(){ return _i; } })(i); }

You’ll almost certainly need to do something similar in your case. (But without seeing a bit more of the context of your code, it’s hard to say exactly what.)